Plant growth media comprising polyurethane foams are known.
EP 939092 discloses open cell polyisocyanurate foams for fixing and supporting plants.
U.S. Pat. No. 3,798,836 discloses a water insoluble open celled foamed polyurethane matrix having dispersed therein thermoplastic particles.
U.S. Pat. No. 3,889,417 makes a hydrophilic foam by reacting a prepolymer with a high amount of water. This foam may be used in horticultural applications. The water index applied is 1300-78000. A similar process has been disclosed in WO 96/16099 wherein no special guidance as to the isocyanate index and the water index has been given; in the examples the water index was more than 1700 and the isocyanate index was less than 6. The prepolymers used are made from polyols having a molecular weight of at least 1000.
U.S. Pat. No. 3,970,618 discloses a substrate for cultivating plants which substrate is a low density (18 kg/m3) hydrophilic polyurethane foam made by reacting a polyisocyanate and a polyol at a low NCO index, the polyol having an hydroxyl value of 900-1800 and the polyisocyanate being a toluene diisocyanate (TDI) type polyisocyanate.
U.S. Pat. No. 5,155,931 uses a foam as plant mat, which foam is made by reacting an organic isocyanate, which preferably is TDI, and a polyol at an NCO-index of 90-120.
U.S. Pat. No. 6,479,433 discloses a horticultural growing medium made by reacting a prepolymer and water in the presence of a selected filler material.
WO2011/042284 describes flexible polyurethane foams for use as plant substrates. These foams have a density of 25-70 kg/m3, a compression load deflection at 40% (CLD) of 5-15 kPa and a volume increase at water saturation of at most 25%. These foams are made at a low isocyanate index of 20-70 and at a high water index of 200-400 which means that there is a huge competition between the isocyanate reactive components in the formulation to react with the available isocyanate components which may lead to frothing (of the draining water) when the foam is used as substrate material.
U.S. Pat. No. 6,460,473 and EP 823203 disclose the use of low density, flexible polyurethane foam for raising rice seedlings.
In WO 2010/099431 a hydroponic apparatus has been disclosed for screening plants. Amongst others rice has been mentioned and polyurethane may be used as germination substrate.
The above cited prior art does not touch upon serious environmental problems associated with the traditional way of rice cultivation wherein methane gas is liberated from the cultivation environment and a solution to reduce the emission of methane gas.
Further the prior art does not disclose polyurethane foam (pieces) suitable for mixing in soil or other natural plant substrates in order to improve the water retention of the soil and hence reduce the amount of water required to grow plants or plant crops (tomatoes, wheat, grass and olives) in e.g. semi-arid and arid climate conditions where the available water is limited while at the same time improve or at least maintaining the yield of the plants.